1. Field of the Invention
This invention pertains to microelectromechanical systems (MEMS) actuators, and more particularly, this invention pertains to microelectromechanical systems actuators that use Joule heating to actuate the actuators.
2. Description of the Related Art
Electrically controlled actuators receive an electrical signal input and provide a mechanical output. The mechanical output is typically a force that is used to move objects. Large, electrically controlled actuators are common in mechanical systems to control valves, pumps, and move objects.
Recent innovations require control of very small components that are formed on semiconductor substrates by conventional semiconductor fabrication processes. In U.S. Pat. No. 5,808,384 a photolithographic process is used to fabricate a micromechanical actuator to control switches, relays, and valves. However, the invention of the 5,808,384 patent does not teach a means to move objects toward and away from the plane of the substrate upon which the device is formed.
A bimorph actuator has been formed on a planar substrate to use Joule heating to induce different rates of thermal expansion in different portions of the actuator thereby creating mechanical movement. This actuator has been limited to motion in the plane of the substrate, i.e., parallel to the substrate surface. Accordingly, such actuators can not be used to move an object toward, and away from, the plane of the substrate without using complicated mechanical linkages that convert motion in the plane of the substrate to motion out of the plane of the substrate.
In U.S. Pat. No. 5,867,297 the inventor discloses a microelectromechanical device to oscillate a mirror for deflecting a laser beam to read bar codes. The mirror is substantially upright and moved about a substratemounted hinge, but the device relies upon staples and hinges that are unreliable for repetitive motion. In addition, the mirror is hinged to the substrate and is therefore constrained to move in rotation about a single axis, thereby limiting its utility. Also, some applications require a mirror that is parallel to the substrate on which it is formed and that can move out of the plane of the substrate: a design limitation that can not be accommodated by the teachings of U.S. Pat. No. 5,867,297.
Currently, many applications seek to control a mirror surface to control light energy and move light beams in order to scan bar codes or create and project video images.
In U.S. Pat. No. 5,192,946 a spatial light modulator includes an array of mirrors that are used to create and project a video image onto a screen. The video image is created by directing light onto the mirror array (referred to in U.S. Pat. No. 5,192,946 as a deformable mirror device) and controlling an orientation of the individual mirrors in order to control a path of the light. The screen image comprises many pixel elements that correspond to individual mirrors in the array. Each pixel element is turned on and off by controlling the orientation of the mirrors to reflect the light toward the screen (through intermediate lenses) or away from the screen. The mirror array includes mirrors formed on a semiconductor substrate that are hinged to move between up and down positions in which the light is reflected toward or away from the screen in order to toggle on and off, respectively, the pixel element displayed on the screen. The individual hinged mirrors can only be rotated about one axis, providing only one degree of motion thereby limiting projector design. Also, the hinged motion of the mirrors of this reference is binary; that is, the mirrors are either fully rotated or fully unrotated and can not be partially rotated. Such video image systems are also referred to as digital light processors (DLP).
In order for such video systems to be viable, the mirror arrays must be easy and inexpensive to fabricate in quantity and provide controllable, high speed movement of the mirrors. Another important criteria is that the mirror actuation means be very reliable to withstand millions of actuations without failure. In many designs, semiconductor hinges have proven to have lower reliability than desired. In addition, it is desirable that the mirrors of the mirror array can be moved a sufficient range of motion along several axes of motion to accommodate projector designs. Accordingly, where other factors are equal, actuators that can move a mirror in 2 or 3 degrees of motion and with a range of motion of .+-.30 degrees about each axis of rotation is preferred to an actuator system that can move a mirror a maximum of .+-.10 degrees in only one degree of motion.